The invention relates to a method and an apparatus for compensating for gain changes in an amplifier circuit.
Circuits for processing RF signals, for example in transmission/receiving modules for phase-controlled antennas, normally comprise GaAs radio-frequency modules in the form of monolithically integrated microwave circuits (MMIC). The component characteristics of such RF components are generally temperature-dependent; that is, the electrical characteristics of the RF components vary as a function of the temperature. This temperature dependency of the RF components disadvantageously influences the electrical characteristics of the modules and circuits in which the RF components are installed.
GaAs radio-frequency modules furthermore have a scatter of their optimum monitoring voltage (for example gate voltage), in particular from one wafer to another and from one batch to another. The requirements may vary from about −1.0 volt to 0 volts. The components of a wafer can be operated with an identical gate voltage, although a scatter remains in the RF characteristics from one component to another.
Manual individual adjustment of the circuits or modules is time-consuming and costly, and is unreasonable at a high production rate. It is therefore desirable to design the module or the circuit such that component scatters are automatically corrected (spread compensation), and temperature effects are largely compensated for by simple, temperature-dependent control (temperature compensation).
European patent document EP 1 293 798 B1 discloses a circuit for a transmitting/receiving module which allows the output power to be regulated in a stable form and largely independently of external influences. In this case, the output power of the transmitting/receiving module is controlled by deliberately adjusting the input power of the transmission chain, with a closed-loop control system always keeping the gain of the amplifiers in the transmission chain constant with respect to the gain of the RF transmission signal. If the amplifier gain is constant, the output power is known when the input power is set with an amplifier with variable gain in the transmitting/receiving module.
One object of the present invention is to provide a method which makes it possible to compensate for component scatters and temperature effects in RF components.
A further object is to provide an apparatus for compensating for gain changes in amplifier circuits.
These and other objects and advantages are achieved by the compensation apparatus according to the invention, which comprises memory devices for permanent storage of values for producing monitoring voltages for the radio-frequency modules and attenuation elements, and electrical circuits having an input and an output for producing a temperature-dependent output voltage taking account of a temperature-dependent input voltage UACT(T), which is applied to the electrical circuits, and a value which is stored in the memory devices, wherein the stored value corresponds to an individual monitoring voltage Uopt for setting the optimum operating point of the radio-frequency modules.
Fundamentally, the expression “attenuation element” in the following text means an amplifier with variable gain, and a “radio-frequency module” means a monolithically integrated microwave circuit (MMIC).
In the method according to the invention, a radio-frequency module is driven with a first temperature-dependent monitoring voltage UHF(T), and an attenuation element with a second temperature-dependent monitoring voltage UVG(T). The first temperature-dependent monitoring voltage UHF(T) is in this case produced by applying a temperature dependency to an individual monitoring voltage Uopt, which is predetermined for a predetermined temperature for a radio-frequency module, in order to set the optimum operating point of the radio-frequency module. Correspondingly, the second temperature-dependent monitoring voltage UVG(T) is produced by applying a temperature dependency to a predetermined monitoring voltage UVG—T for the attenuation element. The monitoring voltage UVG—T is determined by setting the monitoring voltage UVG—T in an iteration method such that the output power of the amplifier circuit reaches a predeterminable level at a constant input power.
The first temperature-dependent monitoring voltage UHF(T) is based on a voltage stated by the manufacturer of the radio-frequency component. This voltage is individual for the respectively produced wafer and indicates the individual monitoring voltage Uopt for setting the optimum operating point for a radio-frequency module in this wafer.
A data word which corresponds to this voltage (referred to as an individual monitoring voltage Uopt) is expediently read to a first memory device associated with the radio-frequency module. This data word is expediently read and stored once, and permanently, when the method is first carried out.
The scatter of the characteristics of the radio-frequency components is determined by RF measurement and is compensated for by means of a voltage-controlled attenuation element, by varying the voltage UVG—T on the attenuation element in an iteration method, and thus setting the gain such that the desired gain is achieved between the input and the output of the amplifier circuit. This measurement is expediently carried out at a fixed temperature of, for example, 25° C. The data word which corresponds to this determined monitoring voltage UVG—T is expediently read to a second memory device, which is associated with the attenuation element. The reading and storage of the value in the second memory device are carried out once and permanently when the method is first carried out.
The amplifier circuit may contain a chain of radio-frequency modules and/or attenuation elements. In this case, the data word for the individual monitoring voltage Uopt is stored for each radio-frequency module, and the data work for the determined monitoring voltage UVG—T is stored for each attenuation element. The storage process is in this case carried out in memory devices, wherein each radio-frequency module and each attenuation element in each case has a respectively associated memory device.
According to the invention, a radio-frequency module is driven with a temperature-dependent monitoring voltage UHF(T), with a temperature-dependent voltage being produced taking account of the stored data work for the monitoring voltage Uopt. Like the radio-frequency module, the attenuation element, which may expediently also be a variable-gain amplifier, is also driven with a temperature-dependent monitoring voltage UVG—T. In this case, a temperature-dependent voltage is produced taking account of the stored data word for the monitoring voltage UVG—T determined by the iteration method.
An electrical circuit with one input and one output is provided for each RF module and each attenuation element, and a temperature-dependent input signal UACT(T) is applied to the input of this electrical circuit. The input signal UACT(T) is expediently produced by means of a diode as a temperature sensor, and an operational amplifier connected downstream from the diode. At the output, the electrical circuit produces a temperature-dependent monitoring voltage, wherein a temperature-dependency is applied to a voltage as a function of the input signal UACT(T). This voltage in this case corresponds in each case to the value stored for one RF component or attenuation element. This temperature dependency may, for example, have a linear, square, exponential or polynomial profile.
Temperature-dependent changes in the characteristics of the radio-frequency modules are compensated for by the temperature dependency of the monitoring voltage UHF(T).
By way of example, radio-frequency components may be RF amplifier stages which are driven with a gate voltage or base voltage. For physical reasons, the gain of an amplifier stage changes by about 0.01 dB/K. By way of example, for 5 stages and a temperature change of 100 K would cause a gain change of about 5 dB (linear: factor 3). About 1/10 of this value or less is desirable. This problem is solved by temperature-dependent attenuation of the voltage-controlled attenuation element, particularly so as to compensate for the temperature-dependent gain change of the radio-frequency components.
In one advantageous refinement of the invention, the memory devices and electrical circuits are integrated in a digital potentiometer, which has one input, and one output for each radio-frequency module and each attenuation element. The temperature-dependent input voltage UACT(T) is applied to the input, and the appropriate monitoring voltage for the radio-frequency component or attenuation element which is connected to the respective output is produced at the output. The values of the voltage dividers (division ratio) which lead to the monitoring voltages Uopt for the radio-frequency components and to the monitoring voltages UVG—T for the attenuation elements are expediently digitally stored in the memory device of the digital potentiometer.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.